This invention relates to a method and a device for checking a screwed connection, and it can be used preferably to check a screwed connection in industrial production, in workshop operations and for monitoring in plant engineering in cases where there are high safety requirements. This invention also relates to special screws for carrying out this method.
It is known in the state of the art that electronically controlled screw systems are being used to an ever increasing extent in industrial production; these screw systems tighten the screws at a predetermined torque and press the parts to be screwed together at a predetermined force. This force is known as the initial tension force or prestressing force and it occurs due to the change in length of the screw shaft, which is referred to below as elongation.
In many applications, it is necessary to determine the prestress force with a high accuracy after production of the connection. Thus, the quality of the connection can be tested by customer service, for example, without having to release the connection and establish it again.
It is known from the state of the art that the prestress force of a screwed connection can be tested indirectly by measuring the releasing torque or the tightening torque. However, these methods are very inaccurate due to the fluctuations in friction in the screw thread and under the screw head and due to the alternation between adhesive friction and sliding friction; furthermore, the prestress force changes during the testing process.
To reduce the influence of friction in testing, methods that determine the prestress force of the screwed connection by measuring the length of the screw are known. To determine the prestress force from the length of a screw, the exact starting length of the unstressed screw must be known. However, this is impossible in mass production in practice due to the great screw tolerances, or it is possible only at a very great expense.
Another possibility of checking the prestress force is by directing an ultrasonic beam into the screw and measuring the transit time of the ultrasonic beam in the screw. By introducing longitudinal and transverse waves, it is possible to determine the prestress force of a screw without knowing the starting length. However, major technical difficulties must be overcome with this method. Thus, for example, this method functions only with screws of a certain geometry, and it is extremely difficult to introduce a transverse wave, and the accuracy of the testing method for determining the prestress force is lower than in the methods used in producing the screwed connection.
The object of this invention is to improve upon the testing of the quality of screwed connections to overcome the problems described above.
This object is achieved with a
checking, with a
checking, and a
screw.
The checking method for screwed connections has the following process steps:
determining the original length of the special screw by measuring the length of the core of the core hole,
determining the length of the special screw in the stressed state by measuring the distance from the screw head to the base of the bore,
comparing the measured length values thus obtained with predetermined data, determining according to predetermined criteria whether the prestress force is sufficient or whether the screw must be tightened.
The main advantage of this method is that the original length of the special screw can be determined by measuring the length of the core of the core hole, because the core is not elongated when tightening the screw. In producing the core hole bore, the core length and the bore depth are always exactly the same, so it is necessary only to compare the length of the bore of the stressed screw with the length of the unstressed core. The difference between these two measured results is the elongation of the screw shaft under prestress, from which the prestress force can then be calculated according to equations with which those skilled in the art are familiar.
Advantageous refinements of this method can be derived from claims 2 through 4.
To measure the depth of the core hole bore and the length of the core, measurement methods known from the related art can be used. In particular, it is also possible to use different methods to measure the depth and the length. It is important only that the measurement methods must meet certain conditions with regard to accuracy and reliability.
The device for checking screwed connections has the following features: a length measuring device which determines the distance between the screw head and the base of the core hole bore and determining the distance between the screw head and the end face of the core of the core bore.
The advantages of this checking device are that the length measuring device available in the state of the art can largely meet all the requirements postulated here and can be acquired inexpensively, so that an optimum configuration of the same or different measurement systems can be compiled, depending on requirements.
According to the invention, the length measurement devices supply electric measurement data which are sent to an electronic data processing unit where the measured data are compared with stored data and a decision is made as to whether the prestress force is sufficient or whether the screw must be tightened further. Further tightening of a screw can be accomplished with techniques known from the related art, i.e., either manually or at another workplace.
According to the invention, a tightening device is also provided, further tightening the screw with a further tightening decision on the part of the electronic data processing system until the intended prestress has been reached. Then, the screwed connection can be checked again with increased accuracy demands.
The screw according to the invention has an annular gap which is covered or filled with a suitable material. Elastic compounds or greases are also suitable, for example.